The invention is based on a fuel injection apparatus as generally described hereinafter. A known fuel injection apparatus of this kind (German Offenlegungsschrift No. 30 02 851) includes a piston supported in a slidable manner in a bore of an auxiliary pump--although solely for the application in which on the one hand a main fuel that is not readily ignited and is supplied by a high-pressure injection pump and on the other hand an igniting fuel pumped by a separate low-pressure feed pump are to be delivered via the hydraulic auxiliary pump to separate injection nozzles for the main fuel and the igniting fuel in a Diesel engine. The piston is prestressed by a spring (see FIG. 5 of this document) and on one end it is acted upon by the pressure of the high-pressure injection pump, while the remote end of the piston is preceded by a work chamber, to which fuel, in this case the igniting fuel, is delivered from the low-pressure feed pump. The piston has a control groove forming a control edge, and the distance between the control edge and the end of the piston remote from the work chamber determines the pilot injection quantity of the igniting fuel.
However, the overlapping times for the pilot and the main injections may be considered problematical, because there is no possibility of arranging the pilot and main injections in a predetermined chronological sequence, or of establishing a definite storage quantity between the two, because of the positive displacement volume of the piston. It is not even possible to prevent another pilot injection from taking place as the piston continues to slide downward in response to the feed pressure of the main fuel. The desired accurate control of the chronological sequence of the pilot and main injections is also difficult because of the dead volumes existing in the many connecting lines and leads to deviations from the specified course of events, especially as a function of rpm.
An apparatus for pilot injection is also known (German Pat. No. 1 252 001) which has a separate small piston disposed axially parallel to and offset from a load piston for the main injection inside a fuel injection valve. A separate supply of low pressure is not provided in this known apparatus; the pilot injection quantity is derived from the supply of fuel for the main injection. As a result, the standing pressure in the pressure line and thus the accuracy of fuel quantity control is unfavorably affected.
Finally, in a further known apparatus (German Offenlegungsschrift No. 28 34 633) for controlling the pilot injection in internal combustion engines, a one-piece control slide displaceable counter to the force of a spring is provided, which with a pronounced intervening relief into a reservoir establishes the various desired connections for the pilot and the main injection. Here, again, the fuel for the pilot injection is diverted from the supply quantity of the injection pump that also furnishes the main injection quantity, so that once again there is a negative effect on the accuracy of fuel quantity control for the main injection quantity.